Sculpting on 2 joined meshes is not working as intended
Let's say that you want to sculpt these two objects together:

First, apply their scale, then apply all their modifiers except Subdivision Surface that you just disable. Keep in mind that when you join objects, the last selected while keep its modifiers (and they will affect the joined meshes) while the others won't:

Join them with CtrlJ, then select all the meshes except one and go into Face &gt; Intersect (Boolean) and in the Operator box choose Boolean &gt; Union:

Set the origin of the object to its geometry, switch to Sculpt mode, enable Dyntopo, it works: